Something I overlooked when this thread was active was that in the early-mid 1950s, Baldwin had done some work on diesel-hydraulic locomotives. An ASME paper presented by J.S. Newton of Baldwin was summarized in the 1954 May issue of Diesel Railway Traction (DRT). This addressed some of the efficiency and utilization issues discussed above.

#8: Multiple operation with locomotives having electric transmissions is desirable.

#9: The drive should permit braking equal to the dynamic brake of the diesel-electric drive where braking power is usually greater than pulling power.

#9 was feasible and was included in the SP/DRGW prototype specifications.

#8 was not included, and here I suspect there was major resistance from the makers of hydraulic transmissions who had decided it was infeasible and perhaps also just did not like the idea. DRGW and SP proved them wrong.

The DRT 1955 June issue included a summary of a paper or presentation by W.B. Gibson of Twin Disc, which in part addressed the desiderata outlined by J.S. Newton of Baldwin.

The author did not seem to be all that keen on the idea of diesel-hydraulic/diesel-electric mixed MU operations, nor was he enthusiastic about the prospects for hydrodynamic braking that was proximate in performance to electrodynamic braking. Yet both turned out to be quite doable.

By the way, this article also contained information on the EMD DH2 prototype additional to that provided elsewhere.

In hindsight, the resistance in some quarters to mixed DH/DE MU operations is difficult to understand. By the mid-1950s, the idea that different makes and models of diesel-electric locomotives could interwork was well established in the USA to the extent of being a norm. Whilst the basic compatibility of EMD and GE power controls was largely happenstance, one could say that Baldwin took the initiative in offering the “compatible” WEMCO XM-781 master controller as an option to its standard pneumatic throttle control, I think from the production start of its road locomotives in the late 1940s. Reconciling the different EMD field loop and GE potential wire dynamic brake controls was done by Fairbanks Morse c.1954. Milwaukee developed its system by which DC electric locomotives could control trailing diesels in 1956, and UP did the same with GTEL-diesel combinations in 1958, although these were one-way systems. Against that background, by 1959-60 DH/DE interworking should surely have looked like a soluble issue simply requiring some detail development work rather than being an uncrossable barrier. Furthermore, GE, with its U25B, had shown that two-way interworking between standard 8-notch and 16-notch control systems was possible, pertinent to the DH/DE case in that by the late 1950s it had been realized that DH locomotives benefitted from multi-notch control, and so were likely to have a higher notch count than standard American DE locomotives.

Still, that history shows that some of the MU initiatives came from the railroad side rather than from the locomotive builders. So it was not so surprising that when the DH locomotive builders ostensibly said, “it can’t be done”, DRGW and SP in 1963 developed their own solutions, and interface unit in the first case and retrofitting with DE-compatible controls (derived from GE’s 16-notch work) in the second case. As a sidebar item, the DRGW interface, which one assumes provided two-way conversion between stepped electric and pneumatic protocols, probably could also have been adapted for use between standard DE and Baldwin air throttle DE locomotives.

The DE/DH MU question thus had been resolved just before Alco built the DH643 fleet for SP.

Yes, I think that unfamiliarity with the North American market was a big factor. And even where some of the differences as compared with European practices were known, their effects on products built in the European way were probably not fully quantifiable short of in-service experience. For example, KM knew that the American roads had staggered rail joints, and did endeavour to compensate for this in the suspension design of the ML4000 prototypes. But the effects tuned out to be of greater magnitude than envisaged, and modifications were needed.

The SP published an SAE paper on the KM prototypes, in which it said, perhaps somewhat optimistically: “At the outset, let me point out that these Krauss-Maffei units are not standard European locomotive design. While they employ European concepts and components, they were built to American standards of strength and construction to meet specifications developed jointly by the mechanical engineering staffs of the railroads and the locomotive builder. Many details are basic American standards which have been employed wherever possible.” That was followed a little later by: “In other words, one might say that these prototype units are European built locomotives of American design.”

Nevertheless the MU compatibility question appears to have involved an element of stubbornness, an entrenched belief that DH/DE mixed MU was something that could not, or at least should not be done. Zenk devoted a chapter to it in his book on the KM Hydraulics, see p.112ff. He deduced that the SP and DRGW chose not to insist upon MU compatibility, and suggested that cost and expediency were the factors involved. From that one can infer that KM did not want to do it, and perhaps had suggested that there would be cost and delay implications were it attempted, with no guarantee of success. When DRGW went ahead with the development of an MU interface, the KM engineers objected, indicating that any such conversion would not receive warranty support.

MU compatibility was part of the SP’s 1962 specification for production units, though, and that requirement appears to have forced the issue.

Alco published an SAE paper on the DH643, which had as its opening: “THE DESIGN AND DEVELOPMENT of a mainline diesel-hydraulic locomotive was undertaken with the potential of greater adhesion, lower possible continuous speed than a diesel-electric for the same horsepower per axle, and lower maintenance on the transmission system. “Design parameters were that: 1. Locomotive must be compatible and operate in all functions in multiple with existing and future domestic diesel-electric locomotives.”

The MU compatibility requirement was number one on a list of eight key items. However, the paper did not describe how that was achieved. It did state that the hydrodynamic brake was controlled in 16 steps, although the paper included what I think was a “stock” diagram of the Voith transmission with an 8-point (3 bit) EP hydrodynamic brake controller. I’d guess that the 16-step unit actually used was the Westinghouse 16-point (4-bit) EP controller first introduced for throttle control on the DB V100 class. Other information on the Voith hydrodynamic brake suggests that at least three forms of braking level control were offered, namely 8-point EP, 16-point EP and continuously variable pneumatic.